Well boreholes are typically drilled in earth formations to produce fluids from one or more of the penetrated formations. The fluids include water and hydrocarbons such as oil and gas. Well boreholes are also drilled in earth formations to dispose waste fluids in selected formations penetrated by the borehole. The boreholes may be lined with tubular commonly referred to as casing. Casing may be steel, although other metals and composites such as fiberglass can be used. The outer surface of the casing and the borehole wall form an annulus, which may be filled with a grouting material such as cement. The casing and cement sheath perform several functions. One function is to provide mechanical support for the borehole and thereby prevent the borehole from collapsing. Another function is to provide hydraulic isolation between formations penetrated by the borehole. The casing can also be used for other functions such as means for conveying borehole valves, packers, pumps, monitoring equipment and the like.
The wall of the casing can be thinned. Corrosion can occur both inside and outside of the casing. Mechanical wear from pump rods and the like can wear the casing from within. Any type of casing wear can affect the casing's ability to provide mechanical strength for the borehole.
Grouting material such as cement filling the casing-borehole annulus hydraulically isolates various formations penetrated by the borehole and casing. If the cement is not properly bonded to the outer surface of the casing, hydraulic isolation is compromised. If the cement does not completely fill the casing-cement annulus, hydraulic isolation is also compromised. Furthermore, if casing corrosion occurs on the outer surface or within, or if wear develops within the casing, holes can form in the casing and hydraulic isolation can once again be compromised.
In view of the brief discussion above, it is apparent that measures of casing wear, casing corrosion, cement bonding and cement distribution behind the casing can be important from economic, operation and safety aspects. These measures will be subsequently referred to as borehole “parameters of interest.”
Measures of one or more of these borehole parameters of interest are useful over the life of the borehole, extending from the time that the borehole is drilled until the time of abandonment. It is therefore economically and operationally desirable to operate equipment for measuring the borehole parameters of interest using a variety of borehole survey or “logging” systems. Such logging systems can comprise multi-conductor logging cable, single conductor logging cable, and production tubing.
Currently ultrasonic methods are employed for measuring many of these parameters of interest. The current ultrasonic method for measuring the acoustic impedance of the material behind casing and the casing thickness involves the acoustic excitation of the casing using an ultrasonic transmitter and sensor to induce resonant reverberations in the pipe, and later receive a returned signal back at the ultrasonic sensor.
As shown in FIG. 3, a typical acoustic waveform measured by a scanning transducer (155) in the prior art is shown. According to the prior art, acoustic waveforms may be recorded from a transducer used to scan the inner circumference of a borehole casing and are preferably digitized in a data processor. The acoustic waveform of FIG. 3 is a plot of the transducer intensity (voltage) as a function of time. For purposes of discussing the prior art shown in FIG. 3, it will be assumed that the waveform is received by a transducer and represents the first returned signal reflection from the inside of the borehole casing. After the transducer is fired, the first reflection occurs at time t1 having an amplitude 312. The time interval between t0 and t1 is defined as the travel time, and is a function of the impedance of the borehole fluid and the distance between the face of the transducer and the inner surface of the borehole casing.
As briefly discussed above, the prior art is directed to inducing a resonance in the borehole casing and subsequently analyzing the frequency characteristics of the resonant frequency reflection. For example, referring back to FIG. 3, the frequency of the reflected waveform in the intermediate frequency time interval 314 is a function of the casing thickness, while the amplitude and rate of decay or “ring down” of the reflected waveform in the ring down time interval 316 is a function of the bonding between the borehole casing and the casing cement, with its value being inversely proportional to the acoustic impedance of the casing cement.
Using the method of inducing reverberations in the casing, the magnitude of the returning resonant reverberations are inversely proportional to the acoustic impedance of the material behind the casing (i.e., cement). However, in very thick casings, for example casings thicker than 1-inch, the ultrasonic sensor has difficulty inducing the reverberations because the resonant frequency of the casing falls too low for typical ultrasonic tools. Examples of using ultrasonic waves for determining “parameters of interests” of a borehole are disclosed in U.S. Pat. App. 2006/0067162 which is incorporated herein in its entirety.
One current solution for determining one or more of the above parameters of interest is to lower the transducer center frequency to cover the frequency band of thick casings. However, when the frequency of the signal becomes too low, the return reverberations are in the noise level and as a result, the impedance value of the material behind the casing and thickness of the casing cannot be quantified.
Additionally, to manufacture transducers having center frequencies low enough to induce resonant reverberations in these thick pipes, the transducers would need to be so thick that the material comprising the transducers (e.g., ceramic) will be weakly polarized or not polarized at all. As a result, it is impractical to manufacture such a transducer. Because lowering the transducer center frequency to cover the frequency band of thicker casings is impractical, accurately measuring the above borehole parameters of interest without having to induce reverberations in the borehole casing is desirable.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.